1. Field of the Invention
The present invention relates to a developing device for visualizing an electrostatic latent image and an image forming apparatus, such as an electrophotographic apparatus or electrostatic recording apparatus, furnished with the developing device, and more particularly, to an image forming apparatus and a developing device thereof, capable of prolonged maintenance of high-quality images produced by means of a one-component developing agent or by using a cleanerless process.
2. Description of the Related Art
In general, if a predetermined amount of paper dust (or dust) or more from a transfer medium is introduced into a developing unit, electric charge on a developing agent is lowered, so that the transfer medium with an image thereon is inevitably subject to ground fogging and irregular image density.
In a developing unit using a one-component developing agent, as compared with one for two-component developing, in particular, the developing agent does not contain a carrier or electrifying material which frequently touches a toner. Therefore, the developing agent must be electrified by seizing only one chance to be put strongly, which greatly influences the invasion of the paper dust. In contact developing, above all, a developing roller is in contact with an electrostatic latent image bearing member (hereinafter referred to as photoconductor), so that the probability of the paper dust straying into the developing unit is high.
Further, the probability of the invasion of the paper dust varies depending on the presence of cleaning means for the photoconductor and the method of cleaning. A two-revolution-for-one-copy process in a cleanerless copying machine and a cleanerless process in a printer are highly susceptible to the paper dust invasion.
Conventionally, in order to prevent the paper dust invasion, dust removing means, such as a fiber brush or Mylar, is brought into contact with a pair of rollers arranged in the middle of a path along which the transfer medium is transported to the photoconductor, so that the paper dust is removed from the rollers by the removing means.
When some of the paper dust is removed so that the fiber brush is saturated with the dust, however, a further effect of paper dust removal cannot be expected of the brush. Although Mylar cannot be saturated with the removed paper dust, it ceases to be able to remove the paper dust if gaps are formed between its contact surface and the roller surfaces due to fine scratches caused by the frictional contact with the rollers.
These problems, which depend on the life of the paper dust removing means, can be solved by only changing the removing means at regular intervals. Actually, however, it is troublesome for a user or serviceman to replace the removing means only. In many cases, therefore, the removing means is fixed to the apparatus body, and a developing device, cleaning device, or toner is improved, instead, to tackle the situation.
As a technique for the cleanerless process, an arrangement for the miniaturization of an image forming apparatus is described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 47-11538. According to this arrangement, the apparatus can be reduced in size by using a unit which combines as a developing device and a cleaning device. In this arrangement, an electrostatic latent image is developed by means of a single developing device as a photoconductive drum passes the developing device for the first time, and a memory image on the drum is removed after transfer operation as the drum passes the developing device for the second time.
However, the memory image is removed after the photoconductive drum starts to pass the developing device for the second time, so that the recording speed is inevitably reduced by half, and a recording area wider than the outer peripheral surface of the drum cannot be obtained. Naturally, therefore, the photoconductive drum requires a relatively large size, so that the apparatus cannot be made small enough.
Described in U.S. Pat. No. 364,926, on the other hand, is a technique for eliminating drawbacks related to speed. This technique uses a developing device which simultaneously develops an electrostatic latent image and removes a developing agent remaining on a photoconductive drum after the preceding transfer cycle as the drum passes for the first time.
A pressurized developing method is known as one of the developing methods which use a one-component developing agent. This method is characterized in that an electrostatic latent image and a toner carrying body (hereinafter referred to as developing roller) are rotated in contact with each other at a relative peripheral speed of approximately zero (as described in U.S. Pat. No. 3,152,012, Jpn. Pat. Appln. KOKAI Publications Nos. 47-13088 and 47-13089, etc.), and has many advantages. Since this method does not require use of any magnetic material, for example, the apparatus can be simplified in construction and reduced in size, and color toners can be used with ease.
In the pressurized developing method described above, developing operation is performed with the developing roller pressed against or in contact with the latent image, so that the developing roller must have elasticity and electrical conductivity. If the photoreceptor is a rigid body, in particular, it is essential to use the elastic developing roller so as to prevent damage to the photoreceptor.
In order to obtain the well-known developing electrode effect and bias effect, it is desired that a conductive layer be provided on or near the surface of the developing roller so that a bias voltage can be applied as required. Since the photoconductive drum and the developing roller are in contact with each other, according to this method, however, it is inevitable that paper dust from transfer paper will be introduced. Since the one-component developing agent, unlike a two-component developing agent, does not contain a carrier, a material for continually frictionally charging the toner, moreover, insufficient electrification attributable to the paper dust has a great influence, resulting in reduction of the electric charge on the toner.
In the cleanerless process, as described above, the next cycle for electrification, electrostatic latent image formation, and developing is carried out with the after-transfer memory image remaining on the photoconductive drum. In the electrifying operation, therefore, the remaining latent image and toner image are electrified superposed, and this toner image is subjected to the next image exposure. Thus, uniform electrification and latent image formation are hindered, and the memory image from the preceding stage appears superposed on the next picture, so that the resulting image is not clear.
This phenomenon is liable to occur particularly when a solid region (in which the developing agent adheres to a wide area) meets with the memory image, such as characters, formed in the preceding stage, so that the developer image, as well as the latent image, often cannot be removed thoroughly. In some cases, therefore, the developer image remains as a memory, and is transferred directly to the paper.
Thus, the conventional recording apparatus cannot enjoy a satisfactory reliability, often failing to produce clear images.
As the paper dust transferred from the transfer medium to the photoconductor accumulates on developer disturbing means, moreover, memory images may be produced even though the capability of the residual toner to become patternless is lowered. Also, the paper dust may be caused to drop from the disturbing means onto the photoconductor or the transfer medium transportation path by vibration produced during emergency operation, such as the removal of jamming, actuation of the photoconductor, etc., thereby entailing irregular electrification and exposure.